1. Field of the Invention
The present invention relates to an imaging system, and more particularly, to a method and apparatus for scanning images such as a document.
2. Description of the Related Art
In recent years, the use of computers for home and business purposes has increased significantly. Computer systems typically include a computer, a monitor, a scanner or all-in-one device, a fax machine, and a printer. Users frequently employ such systems for scanning, modifying, and/or creating various documents. The documents may include personal greeting cards, photographs, pamphlets, flyers, brochures, business presentations, business cards, and other personal or business related documents. Such documents are usually reproduced on a substrate or medium using a personal or business printer, and distributed to various recipients, such as family or friends, or individual business consumers in many different markets around the world. Document imaging apparatuses, such as scanners or all-in-one devices, are often used in this environment to produce electronic files of photographs and other documents. Users typically want to scan a document as quickly as possible.
Typical imaging systems employ a contact image scan bar to scan a document. A row of monochrome sensors is used that is as wide as the page being scanned. A scan line is illuminated using a sequence of red, green, and blue exposures. The line scanner moves continuously while the image information is read and a captured image is created. The continuous movement of the scan bar and sequential reading of the scan lines in different colors often results in color misregistration in the captured image. The artifact from this misregistration is often referred to as color fringing or color misregistration. Color fringing or color misregistration is very clearly seen in images that contain high spatial frequency content, such as text or line pairs. Color fringing evidences itself as rainbow colored outlines along horizontal edges. Users always prefer a faster scan of a document, so there is a tendency to increase the speed at which a document is scanned. The faster the scan bar moves, however, the more pronounced the defect or misregistration becomes visible. Current contact image scan bar systems are unable to eliminate color fringing, and the defect or misregistration is considered to be inseparable from the design.
One suggested method of reducing, but not eliminating, color fringing in a contact image scanner (CIS) has been the use of three rows of sensors in a type of scanner known as a trilinear contact image scanner. Each row of a trilinear scanner has a color filter, i.e., red, green, and blue, applied to the sensors. The image is illuminated using a white light source, such as Cold-Cathode Fluorescent Tube (CCFT), or by multiple LED's. Trilinear scanners reduce color fringing or misregistration along horizontal edges, but do not eliminate such artifacts. Additionally, such scanners suffer from defects. Trilinear scanners exhibit increased color misregistration when a document is scanned at higher speeds. Further, trilinear scanners are more expensive than other scanners because of the high costs of the materials used to construct them. Sensors are often 40% or more of the cost of a contact image scan bar. Since a trilinear scanner utilizes three rows of sensors, the material costs are three times the cost of a scanner with a single scan bar. This, of course results in a significant increase in the cost of a scanner. The data from trilinear scanners must be read out quickly and broken into smaller, parallel data streams for processing. The data are then fed to an analog front end with twelve or more analog channels. The analog front end is complicated and expensive as a result of the circuitry needed to process numerous analog channels of data.